U.S. Pat. No. 3,367,851 (Filreis et al.) discloses an electrically-conductive paper which is capable of uniform heat generation over a large area. The paper may comprise 2-35% by weight of electrically conductive carbonaceous fibers, and the balance may be wood pulp or asbestos of paper making grade. Such a paper provides inexpensive heating panels and food warming trays. Although not mentioned in the patent, the electrical conductivity of the paper decreases with increased moisture content. It is theorized that the moisture causes the paper to swell, thus tending to break the paths of electrical conductivity through the carbonaceous fibers.
To protect it from moisture, a conductive paper may be encapsulated within an electrically-insulating moisture barrier while keeping the moisture content of the paper at a controlled low level. Especially useful in this respect is a laminate of biaxially-oriented polyethylene terephthalate film and polyethylene film. The polyethylene faces of two pieces of the laminate are bonded to the electrically conductive paper and to each other beyond the perimeter of the paper, as shown in U.S. Pat. No. 3,774,299 (Sato et al.). Because that laminate is only partially resistant to water-vapor transmission, changes in ambient humidity can eventually produce changes in the moisture content of the paper, with corresponding changes in its electrical conductivity. The water-vapor transmission has been reduced nearly to zero by covering the electrically-insulating moisture barrier with metal foil, but this has significantly increased the complexity of the manufacturing process and also the cost of raw materials.
The Sato patent suggests that the electrically-conductive paper may be made from either "natural or synthetic fibers such as cellulosic fibers, polyvinyl alcohol fibers, polyamide fibers, acrylic fibers, . . . " etc. Col. 3, lines 10-17). U.S. Pat. No. 3,265,667 (DeFries et al.) concerns a similar conductive paper using "any desirable mixture of fibers either synthetic or natural, organic or inorganic . . . " (col. 2, lines 67-69), but without suggesting any specific mixture.
Currently the most economical synthetic fibers for making paper are made by treating polyolefin fibers to make them readily dispersible in water. The following U.S. Pat. Nos. assigned to Crown Zellerbach Corporation disclose various processes: 3,743,570; 3,787,256; 3,848,027 and 3,891,499. Such treated, water dispersible polyolefins are known as "synthetic wood pulp", hereinafter called "SWP". Crown Zellerbach currently sells SWP in several commercial grades such as E-400, E-600, etc.
The aforementioned patents are silent as to the effect upon electrical conductivity of changing from natural fibers such as wood pulp to synthetic fibers such as SWP. However, it would be expected that an electrically-conductive paper made from SWP, which absorbs very little moisture, would experience virtually no swelling or shrinkage with changes in ambient humidity. Hence, one would expect its electrical conductivity to remain approximately constant at all levels of ambient humidity.
The inventor is not aware of any electrically-conductive paper on the market based on a synthetic fiber. When the inventor made such a paper in the laboratory from SWP, it surprisingly exhibited significantly increased electrical conductivity with increases in ambient humidity.